1. Field of Invention
The present invention relates to the field of marine seismic data acquisition systems and methods of using same. More specifically, the invention relates to systems and methods for active steering of marine seismic sources to maintain inline position of the seismic sources.
2. Related Art
The performance of a marine seismic acquisition survey typically involves one or more vessels towing at least one seismic streamer through a body of water believed to overlie one or more hydrocarbon-bearing formations. In order to perform a 3-D marine seismic acquisition survey, an array of marine seismic streamers, each typically several thousand meters long and containing a large number of hydrophones and associated electronic equipment distributed along its length, is towed at about 5 knots behind a seismic survey vessel. The vessel also tows one or more seismic sources suitable for use in water, typically air guns. Acoustic signals, or “shots,” produced by the seismic sources are directed down through the water into the earth beneath, where they are reflected from the various strata. The reflected signals are received by the hydrophones, or receivers, carried in the streamers, digitized, and then transmitted to the seismic survey vessel where the digitized signals are recorded and at least partially processed with the ultimate aim of building up a representation of the earth strata in the area being surveyed. Often two or more sets of seismic data signals are obtained from the same subsurface area. These sets of seismic data signals may be obtained, for instance, by conducting two or more seismic surveys over the same subsurface area at different times, typically with time lapses between the seismic surveys varying between a few months and a few years. In some cases, the seismic data signals will be acquired to monitor changes in subsurface reservoirs caused by the production of hydrocarbons. The acquisition and processing of time-lapsed three dimensional seismic data signals over a particular subsurface area (commonly referred to in the industry as “4-D” seismic data) has emerged in the last decade or so as an important new seismic prospecting methodology. When conducting repeated surveys, ideally one wants to repeat all source and receiver positions from the base or previous survey. In practice, this is hard to achieve for the entire survey area due to the different environmental conditions encountered in different surveys. Varying currents, both spatially and in time, are the main environmental contributor.
When conducting surveys today, a reference point at the vessel is steered automatically to be at a certain cross line distance from a given pre-plot track. A controller may be used for this, and it controls the autopilot mechanism to achieve its goal. The operator sets manually how far the vessel is to be cross-line from the pre-plot line. Conventionally, seismic source arrays are deployed so that fixed distances are maintained from the towing vessel and from the center of the first seismic recording group of the streamers. During the course of an acquisition line, these distances may change due to several factors including crossline current that introduces an angle to the relation between the line from the towing cable/rope and the seismic line direction, often called feather angle when used to describe the same relation but for streamers. In addition to crossline feather, changes in the inline component of the current may alter the tension on the towing ropes for individual source arrays, which may then stretch or contract, changing the distances from the vessel to the to the source arrays, and from the source arrays to the center of the first seismic recording group.
While adjustments may be made during line change, no mechanism is currently employed to control these separation distances in real time during the course of a marine seismic data acquisition run. This lack of control may result in inline differences between the source coordinates from a base and monitor 4D survey